The present invention relates to electromagnetic field systems, particularly to such systems utilized to provide an image of underground processes, and more particularly to a borehole transmitter utilized in a cross-borehole electromagnetic system to determine the location of such underground processes.
The central problem in petroleum production is the development of a reservoir model that guides the drilling of wells and the management of the field. Ideally, the model provides a three-dimensional numerical representation of the petroleum-bearing rock, properties of the reservoir units and the nature of the boundaries. To construct this model the reservoir engineer has only the detailed data from well logs in a limited number of holes, a geologic concept model, and more recently, structural controls provided by seismic data. The extrapolation of drill hole data to the interwall volume is an area where geophysics can be of great benefit. Using high resolution geophysics to assign physical properties to the model is a relatively new idea which could revolutionize the effectiveness of reservoir simulation.
Seismic velocity and electrical conductivity are both dependent on the porosity, saturation, temperature and anisotropy of typical reservoir rocks and consequently seismic and electrical techniques are a first choice in the search for new reservoir characterization methods. Surface-based 3-D seismic methods have already had a large impact on reservoir engineering by providing detailed maps of the geometry of producing formations and in some cases hydrocarbon distribution. This is a significant departure from their traditional role of finding target structure in an exploration program.
Electrical conductivity has an even more direct relationship to reservoir fluid properties than do seismic parameters because porosity, pore fluid conductivity, saturation, and temperature all determine the conductivity. Electrical logs are indispensable to the reservoir engineer for assessing saturation, pore fluid type and indirectly, permeability. Electrical logging measures the conductivity in the vicinity of the borehole to a radius of a few meters. Means are now at hand to map the conductivity on a reservoir scale and it is this prospect that motivates study and experiments relating to cross-bore electromagnetic methods.
Although seismic methods are relatively mature, the methodology for measuring electrical conductivity on a reservoir scale is in the development stage. Surface low frequency electromagnetic and direct current (dc) resistivity methods have been applied to process monitoring, but such has been limited to identifying the presence and general configuration of relatively shallow processes. High frequency electromagnetic ( greater than 1 MHz) methods have been used in cross-hole configuration since the early eighties, but the low resistivity of most sedimentary formations typically limits the propagation distance of these fields to a few meters.
Cross-hole and borehole-to-surface configurations typically offer improved sensitivity as compared to surface-based schemes. In surface surveys, the fields must first penetrate, with considerable loss of strength, to the target zone, produce a secondary or scattering current, and the fields from these currents again attenuate greatly in returning to the surface. This attenuation obviously limits the sensitivity of small features at depth. A further complication in surface methods is that the near-surface weathered layer is invariably inhomogeneous and thus exerts a strong attenuation and distortion of the fields from deep targets. Getting at least one of the transmitter-receiver pair near the target zone alleviates these problems somewhat but an even greater improvement occurs if both the source and receiver are placed in boreholes. For example, cross-hole dc resistivity surveys have far greater resolution than surface or surface-to-borehole configurations.
It has been established that low frequency cross-hole electromagnetic systems for reservoir scale problems have shown that a low frequency analog of seismic diffraction tomography provides good resolution of interwell features. Thus, as geophysical technology becomes mature, and the emphasis shifted to engineering and front-tracking applications, the geophysicist is directed towards the use of boreholes and electromagnetic systems. By the use of boreholes, one can escape the cultural and geological noise at the surface and also move the sensors much closer to the region of interest. Both of these are necessary to achieve the high sensitivity and high resolution imaging required.
Electromagnetic (EM) systems are typically deployed in ground-based or airborne configurations. Exception to this are the high frequency electromagnetic (HFEM) systems which are often deployed in closely-spaced boreholes. New applications of cross-borehole EM methods include petroleum reservoir characterization, front-tracking in enhanced oil recovery operations, and monitoring of subsurface fluid movements in hazardous waste applications. These applications all require the use of boreholes to achieve the necessary sensitivity. Boreholes in these applications are typically spaced up to 200 meters apart and are drilled through moderate to high conductivity rocks typically found in sedimentary basins. The large spacing of the boreholes and high conductivity makes the high frequency systems impossible to apply due to the high degree of spatial attenuation.
Cross-borehole electromagnetic systems have been developed to provide an image of underground processes; specifically to determine the location of injected plumes of steam used in the enhanced oil recovery process. The electromagnetic data can provide a determination of the subsurface electrical resistivity which is markedly changed by the introduction of steam, and thus mapping of the steam plume by the associated change in electrical resistivity can be accomplished. Such an electromagnetic system utilizes transmitters and receivers located in various boreholes in the oil field involved in enhanced oil recovery process.
While these prior efforts have greatly advanced this field of technology, to effectively apply cross-hole EM requires the development of a higher power/lower frequency system. This need is partially satisfied by the present invention directed to a borehole induction coil transmitter which can operate at frequencies from 1-200 kHz and provides sufficient signal for propagation through rocks typical of oil field strata for more than 400 meters.
It is an object of the invention to provide a borehole induction coil transmitter for through-the-earth imaging.
A further object of the invention is to provide an induction coil transmitter which can operate at frequencies from 1 Hz to 200 kHz.
A further object of the invention is to provide a borehole induction coil transmitter enclosed to survive to depths of 1 km and provides sufficient signal for propagation through oil field strata for more than 400 meters.
Another object of the invention is to provide a transmitter for a cross-borehole electromagnetic system which consists of four basic components, a wound ferrite or mu-metal core, an array of tuning capacitors, a current driver circuit board, and a flux monitor.
Other objects and advantages of the present invention will become apparent from the following description and accompanying drawings. Basically, the invention is a borehole transmitter designed to be an integral part of a cross-borehole electromagnetic system, to provide an image of underground processes, such as to determine the location of injected plumes of steam used in the enhanced oil recovery process.
The transmitter is packaged as a borehole tool and the tool encased in fiberglass, for example, and can survive to depths of 1 km. The transmitter can operate at frequencies from 1-200 kHz, which is sufficient frequency range to provide moderate to high resolution subsurface images in a typical oil field. The transmitter provides sufficient signal for propagation through rocks of a typical oil field strata for more than 400 meters, which allows the field system to be routinely used in an oil field without the need for closely-spaced drill-holes, as in the above-referenced prior high frequency electromagnetic systems. The invention is an induction coil transmitter particularly adapted for cross-hole electromagnetic systems having high power/low frequency. Two embodiments have been developed, one using a ferrite core and operating in the 250 Hz to 200 kHz range; and the other using a mu-metal core and operating in the 1 Hz to 4 kHz range. The first embodiment was designed for relatively shallow environmental applications, and the other embodiment being for deeper, higher temperature boreholes and in steel-cased boreholes, and described in greater detail hereinafter.